If you can play HL2/TF2/CSS/L4D you might be able to run dota 2.
However, if you can run Half-Life 2 Episode 2 my guess is that you can run dota 2 without a problem.
I think its too early to decide, the source engine is getting improved frequently, and until 2011 upgrades and PC prices will go down. So wait till the game is at beta stage and buy a compatible pc/upgrade.

You can go bake your own cookies with ingredients available for everyone to use but it won't taste as good. Sure you might have helped him by suggesting him to add some sugar and reduce the cinnamon, but he decided exactly what went in and what did not go into the mixture. Ultimately, you're eating Icefrog cookies.

If we gonna compare it to valves brand new games it will be something like this:
CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
RAM: 1 GB for XP / 2GB for Vista/7
Video: Direct X 9 compatible video card (Video card must be 256 MB or more and should be a DirectX 9-compatible with support for pixel shader 2.0)

Dota2.com
Q: What kind of hardware requirements will the game have? (by S.Hong)
A: We don’t have an official minimum spec just yet. The goal is to make sure that people with older computers and the PCs at LAN centers will be able to support the game, while also allowing for newer computers to scale well.

GoldSource has a few modification games I love, but that's off-topic >_>

Quote:

Originally Posted by SMOKEDorc

If we gonna compare it to valves brand new games it will be something like this:
CPU: 3.0 GHz P4, Dual Core 2.0 or AMD64X2 (or higher)
RAM: 1 GB for XP / 2GB for Vista/7
Video: Direct X 9 compatible video card (Video card must be 256 MB or more and should be a DirectX 9-compatible with support for pixel shader 2.0)

My PC can run even Crysis 2, but I don't like that sound quad cores make when you give them good pressure >_>

Don't see why the requirements would be much higher than what u need to run cs on the source engine. Unless they are using new graphics engine ready for game to be release afterwards like half-life ep3 etc...

If you can cs-source/halflife 2 good settings i think you would be able to play the game well eneough.

Also Icefrog made point that he doesn't want too much eye candy on screen as it gets in the way of gameplay and hes absolutly right games like HoN is a good example. I still have a hard time picking out the terrain paths properly in the heat of the moment. lol

protip: try tf2 because its free. that runs the source engine so that will give you the best idea (comparably) of the requirements for dota 2

TF2 is a bad example.
People should try Alien Swarm which is also free. It runs on a more recent version of the Source engine and since it features effects and camera settings that will be used in DOTA 2, it should be a better way to test your machine.